The present invention relates to digital radio communications using time-division multiple access (TDMA).
In a TDMA system, a communication channel is divided into successive radio frames and each radio frame is divided into a plurality of timeslots. Multiplexed channels are formed over the communication channel by allocating the timeslots in each frame.
A TDMA radio system of widespread use is the so-called DECT system (xe2x80x9cDigital Enhanced Cordless Telecommunicationsxe2x80x9d), which has been standardized by the European Telecommunications Standards Institute (ETSI). The DECT technology is used in various sectors, including residential telephony, business communications, and public access to other networks.
The DECT operating frequency band is 1,880-1,900 MHz. Each communication channel (one carrier frequency, or more if frequency hopping is provided) can support up to 12 full duplex channels, the frame duration being 10 ms. In a typical application, each channel has an information bit rate of 32 kbit/s in each direction, suitable for transmitting, e.g., a digital speech signal produced by an adaptive differential pulse code modulation (ADPCM) source. Due to the TDMA operation, the 32 kbit/s bit stream is split into 320-bit sequences distributed into the allocated timeslots of the successive frames, along with system and signaling information, the instantaneous bit rate within the timeslots being 1.152 MHz. Gaussian frequency shift keying (GFSK) is used to form the modulated radio signal bursts from those bit sequences.
Owing to the ETSI standardization, various manufacturers have designed DECT components and produce them at relatively low cost. In particular, the burst mode controller (BMC) is the digital circuitry used in the DECT physical layer to control the production of the radio signal from the different bit streams to be transmitted. All details about the DECT physical layer may be found in the ETSI Standard EN 300 175-2, Digital Enhanced Cordless Telecommunications (DECT); Common Interface (CI); Part 2: Physical Layer (PHL), version 1.4.2, June 1999, which is incorporated herein by reference.
FIG. 1 is an illustration of the conventional DECT frame structure. There are 24 timeslots per frame, i.e. 12 slots for the downlink (fixed part to portable part) followed by 12 slots for the uplink (portable part to fixed part). Each timeslot is 416.7 xcexcs long, which represents 480 bit periods. In a typical embodiment, each timeslot may support the transmission of a signal burst obtained by modulating a sequence of 440 bits (381.9 ps) including 48 synchronization bits (S-field), 388 information bits (D-field) and 4 additional bits (Z-field). The D-field includes 64 bits for signaling (A-field), 320 bits for traffic (B-field), e.g. from the ADPCM coder, and 4 additional bits for error detection (X-field). A guard time of 34.7 xcexcs (40 bit periods) is left after the burst at the end of the time slot to account for the propagation time between the FP end the PP and to enable switching of the carrier frequency if required. In another possible timeslot format, the A-field has only 32 bits, whereby the guard time is longer (56 bit periods).
In a number of applications, the spectral bandwidth, which can be used about a particular carrier frequency, is narrower than what has been provided for in the DECT standards. In particular, the United States Federal Communications Commission (FCC) has issued spectral specifications for wireless telecommunications in the 2.4 GHz range. Those specification require for each channel a maximum spectral bandwidth of about 1 MHz. When such requirement applies, the DECT system as it exists cannot be simply transposed because of the aforesaid 1.152 MHz instantaneous bit rate in the timeslots, which leads to spectral bandwidths significantly broader than 1 MHz.
For such applications, it would be desirable to use DECT components, whose design is well adapted to cordless applications and which are available at relatively low cost. In order to adapt DECT components to those requirements, a possibility is to use a quartz crystal having a lower oscillation frequency as a time reference by the BMC. Reducing the quartz frequency by a factor xcex1 (xcex1 1) reduces the bit rate within the timeslots to 1.152/xcex1MHz. A problem with this approach is that the timeslot pertaining to each channel, which contains 320 traffic bits, occurs every 10xc3x97xcex1ms, so that the rate of the input bit stream has to be changed from 32 kbit/s to 32/xcex1kbit/s. This implies the need to provide data and clock conversion for synchronization with other units. Changing the output bit rate of an ADPCM coder from 32 kbit/s to 32/xcex1kbit/s corresponds to changing the sampling frequency of the speech signal from 8 to 8/xcex1kHz. Therefore, the above-mentioned conversion implies converting the coded speech signal into analog form and re-sampling it at 8/xcex1 or 8 kHz. Such conversion significantly increases the complexity of the system. In addition, it is very difficult to synchronize several base stations together (as required in the DECT system in order to support the handover feature) when such base stations are connected to a switching system, e.g. PBX, because such systems typically transport standard 32 kbit/s ADPCM signals, and use a corresponding 8 kHz reference clock for synchronization.
Another possibility, which overcomes the above problems, is to divide the channel bit rate by an integer number (e.g. 2 or 3), while multiplying the timeslot duration by a corresponding number and keeping the 10 ms duration of the frames (with correspondingly less slots). Although this solution is simpler, it has the drawbacks of significantly increasing the power consumption of the portable units and decreasing the number of channels available for multiplexing.
An object of the present invention is to propose another, more convenient, approach to adapt the DECT technology to narrower spectral bandwidths.
A radio station according to the invention comprises DECT circuitry for processing at least one bit stream to be transmitted over a communication channel comprising successive 10 ms radio frames each having a plurality of timeslots, each bit stream being allocated one of said plurality of timeslots in each frame. The DECT circuitry comprises means for extracting bit sequences from each bit stream and means for modulating each of said bit sequences to form a respective radio signal burst inserted into a timeslot allocated to said bit stream in accordance with DECT formats. Each bit stream has a first bit rate controlled by a first clock signal and each of said bit sequences has a second bit rate controlled by a second clock signal. Clock generation means are provided for producing the first and second clock signals having frequencies adapted for said first and second bit rate to be substantially 32 and 1.024 kbit/s, respectively. The number of transmit timeslots per frame will be at most 10, preferably 9 or 10.